Automotive transmissions typically use automatic transmission fluid for a variety of purposes including lubrication, heat transfer, and hydraulic actuation of shift elements. As the fluid circulates, it can pick up small particles. If not removed, these particles can damage various transmission components. Therefore, it is common to circulate the fluid through a filter. Various types of filtration media may be used. A screen mesh is only effective in removing relatively large particles. Therefore, it is common to use depth filter media. Depth filter media, unlike a screen mesh, retain particles throughout the media, rather than just on the surface.
As fluid flows through filtration media, some pressure drop occurs. The pressure drop increases as the flow rate per unit area increases. If the resistance is excessive, then the transmission pump will not circulate a sufficient quantity of fluid or the transmission pump may cavitate. This problem is most acute when the transmission is cold causing the fluid to have higher viscosity. Different types of filtration media offer different degrees of flow resistance per unit area. Typically, types of filtration media that are more effective in removing small particles tend to have higher resistance per unit area. Therefore, the design of a transmission filter assembly requires some compromise between filtration effectiveness and flow resistance. Flow resistance can be reduced without decreasing filtration effectiveness by increasing the area of filtration media. However, the space available for the filter assembly is limited. To package a large area of filtration media in a small filter housing, the media may be pleated.